Document EP-A-0 505 254 shows a circuit of that type which, in the example given, has a reversible pump feeding four motors, each of which is coupled to a respective wheel. Speed sensors are associated with each of the wheels, and the measured speeds are transmitted to a processing device which controls constrictions disposed one per motor on one of the main ducts of each motor. When it is detected that one of the motors is racing, indicative of a skid situation, the through section of the constriction disposed on the main duct of that motor is reduced to cause head loss and to reduce the speed of the motor.
In general, the skid control system is useful under difficult driving conditions, in particular when the vehicle is traveling over difficult terrain and when skidding might occur on one or more wheels. Conversely, when the vehicle is traveling without any risk of skidding, e.g. on the road, the skid control system is deactivated. The constrictions are at their maximum section, so that the flow-rate of fluid in the main ducts is as high as possible in order to enable high speeds to be achieved.